Researchers recently announced the discovery of an incredibly dangerous bug in the OpenSSL encryption library. That library is used by about two thirds of websites, and many VPNs and other secure communications services.
The problem is in a memory leak that allows an attacker to request heartbeat responses which will contain up to 64KB of memory, and to do so over and over without being detected. This has already been shown to be able to capture the server’s RSA secret key. That is the key used to authenticate communications with the clients, and to encrypt the session keys. Other data could be captured as well, but those keys are really the biggest threat.
An attacker with that key could perfectly impersonate the server, or run man in the middle attacks undetectably.
It is unknown if, or how often, this attack has been run in the wild. It is entirely possible that major players, like national intelligence services, may have known about this for some time, and could have been silently intercepting traffic to certain websites, potentially for over 2 years. We just don’t know. There is a call for researchers to set up test sites to detect this activity going forward, but there is no way to know if it happened in the past.
The solution is non-trivial. All affected services need to install the recently available patch to fix the underlying problem. They then need to address the possibility that their keys have been stolen. All server certificates need to be revoked, so clients will know to reject them, and new certificates created and distributed. This is likely to take time, and many sites will be very slow to respond.